Thousands of surfactants may be used in personal care/personal wash compositions. Among these are included sulfates, carboxylates, sulfonates etc. Formulators are constantly looking for surfactants or surfactant systems which are mild to the skin (measured for example by percent of zein dissolved by the surfactant wherein, the less zein which is solubilized, the milder is the surfactant considered).
One surfactant system which is believed to be mild relative to others is a system comprising alkyl phosphate surfactants. Typically, alkyl phosphates are commercially available as mixtures of mono- and di-alkyl esters and it is common to quote the ratio of mono to dialkyl ester, designated as MAP/DAP (monoalkyl phosphate to di-alkyl phosphate) ratio. Monoalkyl esters are diacids and possess two equivalence points, corresponding successively to the formation of the mono and di-salt with increasing degree of neutralization. Dialkyl esters are monoacids and possess a single equivalence point which corresponds approximately with the formation of the mono-salt in the monoalkyl esters.
Unexpectedly, applicants have found that, when alkyl phosphates are blended with specific weak acid auxiliary surfactants, particularly at defined ratios, the resulting systems are perceptibly more mild than the phosphate system alone. Compositions are also adequately foaming, even at the acidic pHs of the invention. Enhanced mildness is specific to conditions in which the alkyl phosphate is at least partially in the mono-salt form while the auxiliary surfactant is undissociated. In a companion application filed on same date as the subject application, applicants claim compositions where phosphate surfactant is combined with weak acid auxiliary agents (e.g., alcohols) generally.
U.S. Pat. No. 4,139,485 to Imokawa et al. discloses use of alkyl phosphate.
U.S. Pat. No. 6,566,408 to Cotrell et al. discloses compositions comprising alkyl ester salts and amphoteric surfactants.
U.S. Pat. No. 4,758,376 to Hirota et al. (Kao) discloses alkyl phosphate ester surfactants (e.g., mixture of mono- and di-alkyl phosphates) which may be used with auxiliary surfactants. The pH at which the systems are used (e.g., ≧7) are high enough, however, that all surfactants (including auxiliary) are in salt form (i.e., are neutralized). While not wishing to be bound by theory, it is believed that only when the phosphate ester surfactant is neutralized but the auxiliary surfactant is not (because it is too weak an acid to deprotonate) will it be possible to form the necessary complex between MAP/DAP salt and undissociated auxiliary surfactant. When both are in salt form, the complex will not form, or at least not enough will form to significantly enhance mildness. Further, in the reference, auxiliary surfactant is used at low levels.
U.S. Pat. No. 4,526,710 to Fujiwara discloses triethanol ammonium laurate blended with dimethyl amine oxide to improve foaming of MAP/DAP mixtures. Auxiliary surfactant is used in salt form, not in an undissociated form where it can form a complex with MAP and/or DAP salts.
Other references include U.S. Publication No. 2004/0228822 to Khaiat; U.S. Publication No. 2004/0136942 to Yamazaki and U.S. Pat. No. 5,635,970 to Vermeer. In these references, ratio of alkyl phosphate to auxiliary surfactant is always outside specific ranges of claimed invention (i.e., 51:49 to 70:30, preferably 55:45 to 65:35) on upper and/or lower range.
Applicants are aware of no art disclosing the combination of alkyl phosphate ester compositions (e.g., comprising blends of mono- and di-alkyl ester salts) and specifically selected weak acid auxiliary surfactants, wherein said auxiliary surfactants are employed at conditions under which the auxiliary surfactant is undissociated (e.g., retain hydrogen and is not neutralized), the ratio of phosphate surfactant to auxiliary surfactant preferably being close to 1:1 (e.g., 55:45 to 70:30), and pH about 4.5 to 6.5.